The US has carried out airstrikes against artillery used by Islamist militants in northern Iraq, the Pentagon says.

Two F/A-18 aircraft dropped 500-pound laser-guided bombs on a mobile artillery piece near Irbil, the capital of the semi-autonomous Kurdish region.

Rear Admiral John Kirby said in a statement that the Islamist militants had been using the artillery to shell Kurdish forces defending the city, where US personnel are located.

According to military officials, the strike was launched from the US aircraft carrier the USS George HW Bush, which in June was moved to the Gulf in preparation for any possible military action in Iraq.

He vowed to protect American citizens and religious minorities and to prevent a "genocide" by the IS, Sunni fighters who are intent on eradicating non-believers.

Video:Iraq Airstrikes To Stop 'Genocide'

In the hours after the airstrikes, the Federal Aviation Administration (FAA) banned all US civilian flights over Iraq. British Airways also decided to stop flying over the war-torn region.

Other international airlines including Turkish Airlines, Lufthansa and Austrian Airlines have halted their flights to Irbil until Monday.

Some 50,000 residents from the ancient Yazidi community have been forced to leave the northern Iraqi town of Sinjar after the militants overwhelmed Kurdish forces.

Many Yazidis are trapped on Mount Sinjar without food or water and are at risk of starvation as the militants surround the base.

Kurdish media has reported that Peshmerga forces have rescued 11,000 of those trapped in the mountains.

Video:First Aid Reaches Mountain Refugees

On Thursday, the US dropped thousands of gallons of drinking water and 8,000 packaged meals to Yazidis.

The airdrops were performed by one C-17 and two C-130 cargo aircraft escorted by two F/A-18 fighters from an undisclosed air base in the region, the Pentagon said.

Chuck Hagel, the US defence secretary, said the American military has sufficient intelligence resources and assets in place to launch strikes by both manned and unmanned aircraft.

He said that, if the IS moves against Irbil, Baghdad or the refugees trapped on a mountain, "it's pretty clear who they are, and they would be pretty identifiable where our airstrikes could be effective".

In his late-night address on Thursday, Mr Obama said: "America is coming to help".

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Gallery: Thousands Of Iraq Yazidis Displaced

Displaced families from the minority Yazidi community, fleeing the violence in the Iraqi town of Sinjarl, west of Mosul, take refuge in Dohuk province

Mr Obama's announcement reflected the deepest American engagement in Iraq since US troops withdrew in late 2011 after nearly a decade of war.

But the President, who opposed the 2003 invasion, has vowed there would be no return of ground troops to Iraq.

"As commander in chief, I will not allow the United States to be dragged into fighting another war in Iraq," he said.

Defence Secretary Michael Fallon also said Britain was offering help to the Americans in their efforts to provide humanitarian support.

Video:The Spread Of The Islamic Caliphate

Pope Francis is sending a cardinal to Iraq to help thousands of Christians fleeing the militants, the Vatican said.

IS has declared a caliphate - an Islamic state - across parts of Iraq and Syria and wants to bring in a strict version of Islamic law. The militants have swept through more than a dozen towns in recent days.

IS believes the Yazidis, who are followers of a religion derived from Zoroastrianism, are "devil worshippers". The group has issued the Yazidis an ultimatum to convert to Islam, pay a religious fine, flee their homes or face death.